The present invention relates to implantable medical devices and more particularly to an improvement to known methods for providing power from a single external device to a multiplicity of implantable microstimulators implanted within the same patient. Known implantable microstimulators receive power inductively from an external device. The improvement provided by the present invention dynamically adjusts the power reception of individual microstimulators to compensate for differences in the separation of each microstimulator from the external device.
Implantable microdevices have the potential to address a number of useful purposes. These purposes range from pain mitigation to Functional Electrical Stimulation (FES). Some of these applications require that a multiplicity of microdevices be implanted in a single patient. For example, paraplegics and quadriplegics often have muscles capable of functioning, but are paralyzed due to damage to nerves that carry impulses to the muscles. Additionally, individuals afflicted with neuro degenerative diseases such as polio and Amyotrophic Lateral Sclerosis (also known as ALS, or Lou Gehig's disease) may be similarly disabled. Functional Electrical Stimulation provides such individuals with use of their muscles by providing artificial stimulation pulses to the patient's muscles, which stimulation pulses result in a desired movement. However, in order to provide effective use of muscles, a number of microstimulators must stimulate various muscles in the arms or legs of the patient in a coordinated fashion.
Prosthetic devices have been used for some time to provide electrical stimulation to excite muscle, nerve or other tissue. Some of these devices have been large bulky systems providing electrical pulses through conductors extending through the skin. Disadvantageously, complications, including the possibility of infection, arise in the use of stimulators which have conductors extending through the skin.
Other smaller stimulators are implants which are controlled through high-frequency, modulated, RF telemetry signals. An FES system using telemetry signals is set forth in U.S. Pat. No. 4,524,774, issued Jun. 25, 1985 for “Apparatus and Method for the Stimulation of a Human Muscle.” The '774 patent teaches a source of electrical energy, modulated by desired control information, to selectively control and drive numerous, small stimulators, disposed at various locations within the body. Thus, for example, a desired progressive muscular stimulation may be achieved through the successive or simultaneous stimulation of numerous stimulators, directed by a single source of information and energy outside the body.
Many difficulties arise in designing RF powered implantable stimulators which are small in size, and are also capable of receiving sufficient energy and control information to satisfactorily operate without direct connection. A design for a small functionally suitable stimulator, a microstimulator, is taught is U.S. Pat. No. 5,324,316 issued Jun. 28, 1994 for “Implantable Microstimulator,” incorporated herein by reference. The '316 patent teaches all the elements required for successful construction and operation of a microstimulator. The microstimulator is capable of receiving and storing sufficient energy to provide the desired stimulating pulses, and also is able to respond to received control information specifying pulse duration, amplitude and shape. The microstimulator of the '316 patent can also be easily implanted, such as by expulsion through a large gauge hypodermic needle.
A large number of microstimulators may be required to stimulate different nerves or muscles in a coordinated manner to achieve a desired motion, or to treat some other medical condition. Known microdevices may either be continuously powered through an RF signal, or may contain a battery that is periodically recharged through an RF signal. In either case, power is provided inductively from an external device to the implantable microdevices. Further, it is desirable to utilize a single external device to provide power to all of the microdevices in a single patient. When using such a single external device, the distance between the external device and the implantable microdevices may vary greatly.
For distances much shorter than the wavelength of the RF signal (e.g., a 3 MHz signal has a wavelength of 100 meter), the magnetic field decays as 1/r3. The energy received by the microdevice is proportional to the square of the magnetic field strength. Therefore, the energy received by a microdevice, with separation r from the external device, attenuates as 1/r6. As a result, small variations in the separation of individual microdevices from the external device, result in very large variations in the energy received by the microdevices. For example, a first microdevice located half the distance from the external device as a second microdevice receives sixty four times more energy than the second microdevice. The microdevices proximal to the external device may have difficulty dissipating the energy they receive, and overheat as a result.
Known microdevices have a surface area of about 2 cm2. Power dissipation from such a device is about 40 mW/cm2 for a 2 degree centigrade temperature rise, resulting in a capability to safely dissipate 80 mW of power with a 2 degree centigrade temperature increase. A typical microdevice requires about 6 mW of power to charge. Thus, if the ratio of separation of individual microdevices from the external device is two, and the microdevice farther from the external device receives 6 mW, the microdevice nearer the external device receives 384 mW of total power, and therefore 378 mW of excess power. The requirement to dissipate the excess power results in a dangerous increase in the temperature of the microdevice and unacceptable heating of adjacent tissue.
What is therefore needed is a method for limiting the power received by microdevices that are near the external device.